1. Field of the Invention
The present invention relates to a switched-capacitor variable gain amplifier having high voltage gain linearity, and more particularly, to a variable gain amplifier having a limited gain step and small gain nonlinearity in a large gain section.
The present invention is derived from a project entitled “Elements and Module for Ubiquitous Terminal [2006-S-006-02]” conducted as an IT R&D program for the Ministry of Information and Communication/Institute for Information and Technology Advancement (Republic of Korea).
2. Discussion of Related Art
A variable gain amplifier (VGA) is a circuit in which a front end of a signal processing circuit adjusts the amplitude of an input signal to keep it within a dynamic range of the signal processing circuit or to give it a specified value. For example, when an analog voice or image signal is processed in an analog circuit, a VGA may be used to adjust the amplitude of an input signal to prevent saturation of an output signal. Also, when an input digital signal input through a channel is attenuated, which causes considerable loss in a digital circuit, a VGA may be used to amplify the digital signal to comply with specifications.
FIG. 1A is a circuit diagram illustrating a conventional VGA 100 having a switched-capacitor structure, and FIG. 1B illustrates an array of gain capacitors of the VGAs 100 illustrated in FIG. 1A.
Referring to FIG. 1A, the conventional VGA 100 includes an operational amplifier 110 for amplifying a difference between two input voltages, and digital-to-analog converters (DACs) 120A and 120B for adjusting a voltage input to the operational amplifier 110 to adjust an amplification rate.
However, in the VGA 100 having the above structure, in order to obtain a limited gain step, an array of DAC capacitors 120A and 120B having various sizes is required. That is, as illustrated in FIG. 1B, in order to control a voltage gain having a dB-linearity of 9-bit, 10 different capacitors including 9 gain capacitors (C1 to C9) and a scaling capacitor Cb other than a sampling capacitor Cs and a feedback capacitor Cf should be used.
However, the array of different-sized capacitors complicates circuit design and layout. Also, independently using a sampling capacitor Cs and a feedback capacitor Cf may cause mismatch when an extremely small voltage gain step is implemented. In addition, in a conventional VGA, since a feedback factor of a capacitor array has a small value, more power is required to obtain the same unit gain frequency.
In view of these drawbacks, an exponential gain control variable gain amplifier using a unit capacitor array and a positive feedback method has been suggested.
FIG. 2A is a circuit diagram illustrating a conventional VGA 200 having an exponential gain control structure, and FIG. 2B illustrates a gain capacitor array of the VGA 200 illustrated in FIG. 2A.
Referring to FIGS. 2A and 2B, the VGA 200 having an exponential gain control structure controls actual values of the capacitor array by first order approximation of an exponential function in the variable gain amplifier having the switched-capacitor structure (refer to FIGS. 1A and 1B). Since unit capacitors constitute the capacitor array, a gain step control circuit can be easily obtained, and a relatively large feedback factor can be obtained, so that power consumption can be reduced.
Here, the first order approximation of an exponential function can be represented by the following Equation 1.
                              ⅇ                      2            ⁢            x                          ≃                              1            +            x                                1            -            x                                              [                  Equation          ⁢                                          ⁢          1                ]            
According to Equation 1, to implement a function of the form (1+x)/(1−x), a value of a feedback variable capacitor Cx can be represented as a negative value (−x).
For this purpose, the VGA 200 having the exponential gain control structure should have a variable capacitor Cx connected to a negative output voltage −Vout rather than to a positive output voltage +Vout, so that the variable capacitor Cx has a negative value at an input part of the operational amplifier 210. That is, the variable capacitor Cx having a negative value necessary for first order approximation of an exponential function can be implemented through a capacitor array having the positive feedback structure.
The voltage gain of the VGA 200 having the exponential gain control structure can be represented by the following Equation 2:
                              A          v                =                                            C              a                        +                          C              x                                                          C              a                        -                          C              x                                                          [                  Equation          ⁢                                          ⁢          2                ]            
According to Equation 2, in order to obtain a gain of 0 dB or lower, i.e., a gain between 0 and 1, the variable capacitor Cx should have a negative capacitance.
However, since no capacitor has a negative capacitance, it is impossible to implement a voltage gain of 0 dB or lower using the VGA 200 having the exponential gain control structure, and thus a large input voltage may result in a malfunction.